Synchronous rectification is a technique for improving the efficiency of rectification in power supplies by replacing diodes with actively controlled switches such as transistors (e.g., power MOSFETs). The constant voltage drop of a standard p-n junction diode often used for power supply rectification is typically between 0.7 V and 1.7 V causing significant power loss in the diode. One solution to this problem replaces standard silicon diodes with Schottky diodes, which exhibit very low voltage drops (e.g., as low as 0.3 volts). However, even Schottky rectifiers can be significantly more lossy than the synchronous type at high current values and low voltages.
When addressing very low-voltage converters, such as a buck converter power supply for a computer (with a voltage output of five volts or less, and many amperes of output current), Schottky rectification does not provide adequate efficiency. In such applications, synchronous rectification becomes necessary. Replacing the diode with an actively controlled switching element such as a MOSFET is the main factor of synchronous rectification. The MOSFETs have a constant very low resistance when conducting, referred to as on-resistance (RDS(on)). They can be made with an on-resistance as low as ten mΩ or even lower. The voltage drop across the transistor is then much lower versus a diode rectifier, meaning a reduction in power loss and a gain in efficiency.
The control circuitry for synchronous rectification often uses comparators to sense the voltage of the input AC and activates the transistors at the correct times to allow current to flow in the desired direction to produce DC voltages. The timing is important, as a short circuit across the input power must be avoided and can easily be caused by one transistor turning on before another has turned off. One issue with such type of control circuits is that in order to turn the rectifiers off at the correct time, current may have to be directly sensed at the output of the power supply circuit which can be a problem for some applications.